A circuit board (sometimes referred to as a printed circuit board or a printed wiring board) is the basic building block for interconnecting electronic devices in a system. Electronic devices, usually integrated circuit (IC) devices, are mounted onto the circuit boards using a number of mounting mechanisms, such as by use of connectors or by directly mounting the devices onto a surface of the circuit board. A circuit board includes the wiring required to interconnect the devices electrically, and the circuit board also provides the primary support for the devices.
A circuit board typically includes multiple layers, which can range from two layers to fifty or more layers, as examples. Some layers are used for signal transmission, while other layers are used for power and ground reference planes. A reference plane is a plane that is connected to a reference voltage, such as ground, a three-volt voltage, a five-volt voltage, or some other power supply voltage. Some reference planes may even be coupled to more than one voltage.
Typically, a transmission line on a circuit board is formed by a signal trace (routed on a signal layer of the circuit board) and a reference plane (that is provided in a reference plane layer of the circuit board). A circuit board is designed to achieve transmission lines with a target characteristic impedance, which is usually 50 ohms or 75 ohms for single-ended lines and 100–150 ohms for differential lines. The characteristic impedance of a transmission line is dependent on several factors: the inductance of the conductors that make up the transmission line, the dielectric medium, the distance to a reference plane, and the capacitance between the conductors.
Vias are used to electrically connect signal traces at different layers. A via is an electrical connection that is run through multiple layers of the circuit board to complete a signal path using two or more routing layers. Typically, the via is run generally perpendicularly to the main surface of the circuit board. In forming a via, some amount of dielectric material is removed by drilling, laser, or other methods. Next, an electrically conductive metal, usually copper, is flowed or plated into the void to provide the electrical connection between signal lines at different layers.
To avoid shorting problems, minimum clearances are defined between each via and the surrounding signal wires or reference planes. To maximize the density of wires that can be run through each layer of the circuit board, it is desirable for the spacing between the signal lines and a via to be the minimum possible while still avoiding short-circuit problems. Conventional circuit board design and manufacturing methods require minimum clearance dimensions to be maintained equally on all board layers, regardless of whether the layer is a signal layer or a power reference layer.
Because vias are configured differently than signal traces, the characteristic impedance of a via is often different than that of a transmission line. Usually, the characteristic impedance of a via is less than the characteristic impedance of a signal transmission lines on the circuit board. As a result, if a signal path is routed through one or more vias between different layers of the circuit board, impedance discontinuities are introduced by the presence of the vias. For signals having low frequencies, the feature dimension of a via is usually much less than a wavelength of each signal. Therefore, such a low-frequency signal usually does not experience effects of impedance discontinuity introduced by the vias. However, as the frequency of the signal increases, the feature dimension of the via becomes a significant portion of, or is even larger than, a wavelength of the signal. The impedance discontinuity introduced by vias along a signal path can cause reflections, which degrades signal quality and performance in a system.
Another factor that degrades signal performance is the presence of stubs formed by unused end portions of vias. A through-hole via extends through multiple signal layers of a circuit board. In a circuit board having many signal layers, it is sometimes the case that a via connects signal traces on two signal layers in close proximity to each other, leaving portion(s) of the via extending to other layers unused. In such a situation, the unused portion(s) of the via form stub(s). The stub(s) cause signal reflections that degrade signal transmission performance.